Along with human herpesvirus-6 (HHV-6) and human herpesvirus-7 (HHV-7), human cytomegalovirus (HCMV) belongs to beta-herpesvirus of Herpesviridae. HCMV is a double-stranded DNA virus that is the largest among Herpesviridae with a diameter of approximately 180 nm, where the wild strain thereof encodes 165 genes with a genome size of approximately 235 kbp (Non-Patent Documents 1 and 2).
HCMV is highly species-specific and does not infect animals other than human but it widely infects human and has affinity for broad types of tissues in human body.
Moreover, once infected, HCMV is not eliminated even after the establishment of immunity in the host and remains lifelong.
Once infected, HCMV remain latent over a lifetime. More than 90% of Japanese adults are already infected and carry the virus, although the virus is rarely activated and rarely produce diseases in healthy people.
However, an immunodeficiency state due to AIDS, due to cancer, after organ transplantation, after bone-marrow transplantation, after hemodialysis or the like could cause reactivation of the latent HCMV, which may cause serious HCMV infection that may be fatal such as interstitial pneumonia, retinitis, gastroenteritis, encephalitis or the like (Non-Patent Documents 3 and 4).
Furthermore, when a pregnant woman experiences primary infection by HCMV during the fetal period, HCMV infection may be transmitted to the fetus from the pregnant woman via the placenta, in which case the fetus may develop congenital CMV infection (congenital cytomegalovirus disease: also called cytomegalic inclusion disease or congenital cytomegalic inclusion disease), and may result in miscarriage, stillbirth or death shortly after birth. Even in the case of survival, it may result in low birth weight, hepatosplenomegaly, jaundice, thrombocytopenic purpura, microcephaly, disorder of mental development, delay of intellectual development, chorioretinitis, hearing impairment or the like. In addition, even during newborn period or infancy, if HCMV antibody transmitted from the mother is insufficient, HCMV infection via birth canal, breast milk, urine, saliva or the like may develop abnormality of hepatic function, interstitial pneumonia, mononucleosis or the like (Non-Patent Documents 3,4 and 5).
HCMV infection has become a worldwide issue and prophylactic and therapeutic drugs that are capable of suppressing onsets of various pathological conditions due to the above-described HCMV, or that are capable of alleviating such pathological conditions are currently under development. Recently, ganciclovir (Non-Patent Documents 6 and 7), foscarnet (Non-Patent Document 8), valganciclovir (Non-Patent Document 9) and the like have been developed as antiviral drugs for suppressing HCMV proliferation.
Ganciclovir is an antiviral drug that blocks synthesis of viral DNA, which is activated to ganciclovir triphosphate in cells and competitively antagonize with deoxyguanosine triphosphate (dGTP), i.e., a substrate of DNA polymerase, thereby inhibiting DNA polymerase. Hence, it is used for treating cytomegalovirus retinitis in an immunodeficiency state, mainly AIDS, or used for suppressing onset of cytomegalovirus retinitis in advanced HIV infection with 100 or less CD4 lymphocytes/mm3, and approved as a pharmaceutical product.
Antiviral agents such as ganciclovir, however, have been reported of various side effects such as hematopoietic disorder, and their mode of use has been very limited as described above.
The most frequently-occurring and attention-requiring side effect among the side effects of ganciclovir is blood disorders associated with bone-marrow suppression, where the numbers of leukocytes, erythrocytes and platelets are abnormally decreased. The early symptoms include fever, sore throat, abnormal sluggishness and bleeding tendency such as bleeding beneath the skin In some cases, it may cause abnormality in the psyconeurotic system , resulting in headache, dizziness, insomnia, difficulty with thinking, feeling of anxiety or the like.
In addition, teratogenesis, mutagenicity and carcinogenicity have been reported in animal experiments, and thus it cannot be used during pregnancy.
Furthermore, although use of ganciclovir in severe cases of congenital HCMV infection is thought to be effective in decreasing onset of neurological aftereffects and improving progression of hearing loss (Non-Patent Document 10), problems of bone-marrow suppression, teratogenesis or carcinogenicity as side effects of ganciclovir still require sufficient and careful consideration.
Valganciclovir is L-valine ester of ganciclovir which is converted to ganciclovir with intestinal and hepatic esterases after oral administration. Accordingly, its mechanism of action and side effects are the same as those of ganciclovir and thus associated with the problems of bone-marrow suppression, teratogenesis and carcinogenicity. In Japan, valganciclovir is approved as a therapeutic drug for cytomegalovirus retinitis in immunodeficiency states, mainly AIDS.
Meanwhile, foscarnet is an analog of pyrophosphoric acid and suppresses HCMV proliferation by directly acting on the pyrophosphoric acid-binding site of DNA polymerase to inhibit DNA polymerase. It is also effective against ganciclovir-resistant HCMV. Main side effects include feeling of sickness, anemia, increase in serum creatinine, vomiting, hypomagnesemia, hypokalemia, abnormal sensation and the like. In particular, shock and kidney damage frequently occur and thus requires careful administration. In Japan, use of foscarnet is permitted only in patients determinably diagnosed to have HCMV retinitis or patients highly and clinically suspicious of HCMV retinitis among the AIDS patients, and not to be used for the purpose of prevention of infection (Non-Patent Document 8).
Thus, development of a drug capable of preventing onset of various diseases due to the above-described HCMV or alleviating symptoms thereof without any side effect has been strongly desired. Under the circumstances, clinical developments of two anti-HCMV antibodies have been carried out, where one is anti-HCMV antibody “C23 (also called TI-23 at the time of development)” described in Patent Document 3 which is an antibody that recognizes the AD2 region of glycoprotein gB, and the other is anti-HCMV antibody “SDZ MSL 109” described in Patent Document 4 which is a monoclonal antibody that recognizes the glycoprotein gH on the surface of the HCMV. Particularly, development of C23 was abandoned along the way despite of its high neutralizing activity, i.e. 50% inhibitory concentration of 0.5 μg/mL as determined by plaque method (Non-Patent Document 11).
For the time being, development of HCMV vaccine has been keenly conducted but no vaccine is still available that can endure clinical use. A formulation of human-derived anti-CMV high-titer gamma globulin was recently developed, which is approved in the United States of its use for preventing the onset of HCMV infection associated with kidney transplantation. However, since an anti-CMV high-titer gamma globulin formulation is a human-derived blood preparation, it has various problems. For example, a mixture of human-derived gamma globulin results in significant lot-to-lot variation in the activity, low activity and limited supply, as well as constant risks such as contamination of unknown pathogenic virus or pathogen.
Accordingly, since a monoclonal antibody (hereinafter, sometimes referred to as an “anti-HCMV antibody”) that binds to HCMV and neutralizes the infectivity thereof (i.e., annihilating the biological activity thereof) can be expected as a prophylactic or therapeutic drug for various pathological conditions caused by HCMV, it is useful, for example, in terms of prevention or treatment strategy against various diseases caused by HCMV in a patient with an immunodeficiency condition.
Specifically, a human-derived anti-HCMV antibody that has strong affinity and high neutralizing capacity against HCMV to annihilate HCMV activity to prevent the onset of the diseases or alleviate the symptoms and that cause no allergic reaction appeared to be effective to be administered as a so-called “antibody drug”.
However, HCMV-inhibiting antibodies reported so far (for example, Patent Documents 1, 2, 3, 4 and 5) were insufficient in terms of affinity, neutralizing capacity and the like against HCMV, and thus they were short of being expected to sufficiently block HCMV bioactivities to prevent onset of various diseases caused by HCMV or to alleviate symptoms thereof.
Therefore, development of an anti-HCMV antibody or an antigen-binding fragment thereof, which is a human monoclonal antibody that does not recognize or respond to a foreign substance, that has excellent affinity, specificity and neutralizing capacity for HCMV, and that can be expected of its use as a prophylactic or therapeutic drug, has been strongly desired.
Lately, in order to suppress HCMV proliferation in vivo, not only the neutralizing activity but also the importance of blocking cell-to-cell infection has been mentioned (Non-Patent Documents 12 and 13), and thus an anti-HCMV antibody that also has an activity to inhibit cell-to-cell infection has been much needed.